The present invention relates to an improved sigma delta modulator. Sigma delta modulators operating at low input values or with DC inputs may suffer from tonal behavior. For example, the output spectrum may show large tones resulting in a degradation in signal-noise ratio (SNR). Dither, which is an intentionally applied form of noise, may be introduced to break such tones. However, the introduction of dither may require a loop filter with additional headroom.
As illustrated in FIG. 1, dither 110 may typically be added in front of a quantizer 130 in a conventional sigma delta modulator. In this manner, the dither noise is shaped by the loop filter 120 and at the output of the modulator 100, there will be little dither power in the signal bandwidth. In this case, however, dither will be added to the feedback loop, also requiring additional headroom in the loop filter 120. For example, in the case of sigma delta modulators with a signal transfer function of unity, the loop filter 120 may only process the quantizer error. If the dither is added to the quantizer input, dither adds directly to the quantizer error requiring increased headroom for the loop filter 120. Increased headroom typically leads to higher power consumption. At low power supplies, power efficient telescopic operational transconductance amplifiers (OTA) may not be used in the loop filter 120. Thus, a two stage amplifier, which requires additional power, may be required to accommodate the larger output swing.